Google Rich Snippets Testing Tool

The Google Rich Snippets Testing Tool can parse several microformats from a given URL and produces reports showing errors and a preview of what properties it detects. It supports a subset of properties on some microformats, however is continuously updated with more support.

rel-lint

validator project

There has been a suggestion to host an "official" microformats validator at microformats.org.

Ideas:

hCard validator is open-source (BSD-licensed) and extensible enough to support other microformats (it's mostly a matter of hooking up XSLT templates that extract microformat data and find errors). The codebase is pretty clean, UI is internationalized.

Optimus validator is not working for the moment

Therefore any page you will check through will return the message: "Microformats at this page have no errors. Congratulations." which is a false message. We will update this message when (and if) Optimus will work again.